1SPHDISTANCE(1) GMT SPHDISTANCE(1)
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6 sphdistance - Make Voronoi distance, node, or nearest-neighbor grid on
7 a sphere
8
10 sphdistance [ table ] -Ggrdfile [ -C ] [ -Ed|n|z[dist] ] [ -Iincre‐
11 ment ] [ -Lunit ] [ -Nnodetable ] [ -Qvoronoi.txt ] [ -Rregion ] [
12 -V[level] ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -hheaders ] [
13 -iflags ] [ -r ] [ -:[i|o] ]
14 Note: No space is allowed between the option flag and the associated
16 arguments.
17
19 sphdistance reads one or more ASCII [or binary] files (or standard
20 input) containing lon, lat and performs the construction of Voronoi
21 polygons. These polygons are then processed to calculate the nearest
22 distance to each node of the lattice and written to the specified grid.
23 The Voronoi algorithm used is STRIPACK. As an option, you may provide
24 pre-calculated Voronoi polygon file in the format written by sphtrian‐
25 gulate, thus bypassing the memory- and time-consuming triangulariza‐
26 tion.
27
29 -Ggrdfile
30 Name of the output grid to hold the computed distances (but see
31 -E for other node value options).
32
34 table One or more ASCII (or binary, see -bi[ncols][type]) data table
35 file(s) holding a number of data columns. If no tables are given
36 then we read from standard input.
37 -C For large data sets you can save some memory (at the expense of
39 more processing) by only storing one form of location coordi‐
40 nates (geographic or Cartesian 3-D vectors) at any given time,
41 translating from one form to the other when necessary [Default
42 keeps both arrays in memory]. Not applicable with -Q.
43 -Ed|n|z[dist]
45 Specify the quantity that should be assigned to the grid nodes.
46 By default we compute distances to the nearest data point [-Ed].
47 Use -En to assign the ID numbers of the Voronoi polygons that
48 each grid node is inside, or use -Ez for a natural near‐
49 est-neighbor grid where we assign all nodes inside the polygon
50 the z-value of the center node. Optionally, append the resam‐
51 pling interval along Voronoi arcs in spherical degrees [1].
52 -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
54 x_inc [and optionally y_inc] is the grid spacing. Optionally,
55 append a suffix modifier. Geographical (degrees) coordinates:
56 Append m to indicate arc minutes or s to indicate arc seconds.
57 If one of the units e, f, k, M, n or u is appended instead, the
58 increment is assumed to be given in meter, foot, km, Mile, nau‐
59 tical mile or US survey foot, respectively, and will be con‐
60 verted to the equivalent degrees longitude at the middle lati‐
61 tude of the region (the conversion depends on PROJ_ELLIPSOID).
62 If y_inc is given but set to 0 it will be reset equal to x_inc;
63 otherwise it will be converted to degrees latitude. All coordi‐
64 nates: If +e is appended then the corresponding max x (east) or
65 y (north) may be slightly adjusted to fit exactly the given
66 increment [by default the increment may be adjusted slightly to
67 fit the given domain]. Finally, instead of giving an increment
68 you may specify the number of nodes desired by appending +n to
69 the supplied integer argument; the increment is then recalcu‐
70 lated from the number of nodes and the domain. The resulting
71 increment value depends on whether you have selected a grid‐
72 line-registered or pixel-registered grid; see App-file-formats
73 for details. Note: if -Rgrdfile is used then the grid spacing
74 has already been initialized; use -I to override the values.
75 -Lunit Specify the unit used for distance calculations. Choose among d
77 (spherical degree), e (m), f (feet), k (km), M (mile), n (nauti‐
78 cal mile) or u survey foot. A spherical approximation is used
79 unless PROJ_ELLIPSOID is set to an actual ellipsoid.
80 -Nnodetable
82 Read the information pertaining to each Voronoi polygon (the
83 unique node lon, lat and polygon area) from a separate file
84 [Default acquires this information from the ASCII segment head‐
85 ers of the output file]. Required if binary input via -Q is
86 used.
87 -Qvoronoi.txt
89 Append the name of a file with pre-calculated Voronoi polygons
90 [Default performs the Voronoi construction on input data]. For
91 binary data -bi you must specify the node information separately
92 (via -N).
93 -Rwest/east/south/north[/zmin/zmax][+r][+uunit]
95 west, east, south, and north specify the region of interest, and
96 you may specify them in decimal degrees or in
97 [±]dd:mm[:ss.xxx][W|E|S|N] format Append +r if lower left and
98 upper right map coordinates are given instead of w/e/s/n. The
99 two shorthands -Rg and -Rd stand for global domain (0/360 and
100 -180/+180 in longitude respectively, with -90/+90 in latitude).
101 Alternatively for grid creation, give Rcodelon/lat/nx/ny, where
102 code is a 2-character combination of L, C, R (for left, center,
103 or right) and T, M, B for top, middle, or bottom. e.g., BL for
104 lower left. This indicates which point on a rectangular region
105 the lon/lat coordinate refers to, and the grid dimensions nx and
106 ny with grid spacings via -I is used to create the corresponding
107 region. Alternatively, specify the name of an existing grid
108 file and the -R settings (and grid spacing, if applicable) are
109 copied from the grid. Appending +uunit expects projected (Carte‐
110 sian) coordinates compatible with chosen -J and we inversely
111 project to determine actual rectangular geographic region. For
112 perspective view (-p), optionally append /zmin/zmax. In case of
113 perspective view (-p), a z-range (zmin, zmax) can be appended to
114 indicate the third dimension. This needs to be done only when
115 using the -Jz option, not when using only the -p option. In the
116 latter case a perspective view of the plane is plotted, with no
117 third dimension.
118 -V[level] (more ...)
120 Select verbosity level [c].
121 -bi[ncols][t] (more ...)
123 Select native binary input. [Default is 2 input columns].
124 -bo[ncols][type] (more ...)
126 Select native binary output. [Default is same as input].
127 -d[i|o]nodata (more ...)
129 Replace input columns that equal nodata with NaN and do the
130 reverse on output.
131 -e[~]"pattern" | -e[~]/regexp/[i] (more ...)
133 Only accept data records that match the given pattern.
134 -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
136 Skip or produce header record(s).
137 -icols[+l][+sscale][+ooffset][,...] (more ...)
139 Select input columns and transformations (0 is first column).
140 -r (more ...)
142 Set pixel node registration [gridline].
143 -:[i|o] (more ...)
145 Swap 1st and 2nd column on input and/or output.
146 -^ or just -
148 Print a short message about the syntax of the command, then
149 exits (NOTE: on Windows just use -).
150 -+ or just +
152 Print an extensive usage (help) message, including the explana‐
153 tion of any module-specific option (but not the GMT common
154 options), then exits.
155 -? or no arguments
157 Print a complete usage (help) message, including the explanation
158 of all options, then exits.
159
161 The ASCII output formats of numerical data are controlled by parameters
162 in your gmt.conf file. Longitude and latitude are formatted according
163 to FORMAT_GEO_OUT, absolute time is under the control of FOR‐
164 MAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point val‐
165 ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for‐
166 mat in effect can lead to loss of precision in ASCII output, which can
167 lead to various problems downstream. If you find the output is not
168 written with enough precision, consider switching to binary output (-bo
169 if available) or specify more decimals using the FORMAT_FLOAT_OUT set‐
170 ting.
171
173 Regardless of the precision of the input data, GMT programs that create
174 grid files will internally hold the grids in 4-byte floating point
175 arrays. This is done to conserve memory and furthermore most if not all
176 real data can be stored using 4-byte floating point values. Data with
177 higher precision (i.e., double precision values) will lose that preci‐
178 sion once GMT operates on the grid or writes out new grids. To limit
179 loss of precision when processing data you should always consider nor‐
180 malizing the data prior to processing.
181
183 To construct Voronoi polygons from the points in the file testdata.txt
184 and then calculate distances from the data to a global 1x1 degree grid,
185 use
186 gmt sphdistance testdata.txt -Rg -I1 -Gglobedist.nc
188 To generate the same grid in two steps using sphtriangulate separately,
190 try
191 gmt sphtriangulate testdata.txt -Qv > voronoi.txt
193 gmt sphdistance -Qvoronoi.txt -Rg -I1 -Gglobedist.nc
194
196 gmt, sphtriangulate, triangulate
197
199 Renka, R, J., 1997, Algorithm 772: STRIPACK: Delaunay Triangulation and
200 Voronoi Diagram on the Surface of a Sphere, AMC Trans. Math. Software,
201 23(3), 416-434.
202
204 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
2055.4.5 Feb 24, 2019 SPHDISTANCE(1)